Light is a kind of electromagnetic
wave. A chart of the electromagnetic wave spectrum is shown in Figure 1. It can be seen
that visible light occupies a very narrow band of the whole spectrum. Typically, their
wavelengths are between 4 x 10-7 m(4,000 Å or 0.4 m m)
to 7 x 10-7 m(7,000 Å or 0.7 m m). ( Å stands
for 'angstrom' and is equal to 10-10 m.)

Figure 1 The Electromagnetic Spectrum

Lights with shorter wavelengths and
hence higher frequencies have more energy than those with longer wavelengths and hence
lower frequencies.

By the use of special sources or
special filters, it is possible to limit the wavelength spread to a small band, typically
of a width of 100 Å. Such lights can be described as 'monochromatic',
meaning of a single colour.